High strength steels



United States Patent 7 3,293,028 HIGH STRENGTH STEELS Arthur R. Elsea, Frank J. Barone, and William R. Warke, Columbus, Ohio, assignors, by mesne assignments, to The Battelle Development Corporation, Columbus, Ohio, a corporation of Delaware No Drawing. Filed July 18, 1962, Ser. No. 210,855 8 Claims. (Cl. 75--123) This invention relates to ultrahigh-strength steel compositions, and, more particularly, to alloys exhibiting 0.2 percent offset yield strengths of 250,000 p.s.i., or greater, combined with relative insensitivity to the presence of inadvertent manufacturing flaws and defects.

In the design of solid-propellant rocket motors, one of the most critical problems is to find suitable ultrahighstrength sheet materials from which fuel containers can be fabricated. It is desirable to use ultrahigh-strength materials for this application so as to reduce the weight of the fuel containers. However, in the past, as the strength level of available engineering materials increased, the ductility decreased, and the sensitivity to notches and other inadvertent stress raisers increased. Furthermore, it is characteristic of all structural materials that they exhibit less ductility in the presence of biaxial or triaxial tensile stresses. Service loads imposed on solid-propellant fuel containers result in fairly severe biaxial tensile stresses, and any type of stress raisers (such as small flaws from the fabrication process, or possibly nonmetallic inclusions or mi-crotracks in the metallic structure) may lead to catastrophic brittle failures at nominal stresses far below the expected strength of the materials as determined by uniaxial load.

It is, therefore, an object of this invention to provide alloys possessing ultrahigh tensile strengths with good ductility. It is a further object of this invention to provide steel alloy materials possessed of ultrahigh strengths together with high fracture strengths in the presence of biaxial and triaxial tensile stress, and in the pressure of small flaws. Other objects and advantages of the present invention will be apparent from the following detailed description thereof.

As has been noted above, for certain applications, uch as rocket motor casings, it is not enough that a material possess ultrahigh strength with ductility; it must also possess what is commonly designated as fracture strength. Fracture strength is a measure of a material's ability to withstand stress in the presence of a notch, crack, or similar small flaw. Although there is no standard way of determining fracture strength, there are several methods that are used. All of these methods are characterized by the fact that they are complex and involved. In all of them fracture strength usually decreases with increasing tensile strength or yield strength, for a given material.

Using one of the methods for determining fracture strength, it has been demonstrated that compositions according to this invention have fracture strengths of 250,000-260,000 p.s.i. with yield strengths of 260,000- 275,000 p.s.i. SAE 4340, one of the least notch-sensitive high strength steels, by the same method of determination, has a fracture strength of 200,000 p.s.i. at a yield strength of 215,000 p.s.i. Thus, the compositions of this invention possess fracture strengths 50,000 to 60,000 p.s.i. higher than the better prior art high strength steels, and, they havethese higher fracture strengths at 45,000 to 60,000 p.s.i. higher yield strengths than the optimum strength level for the prior art steels. This combined high fracture strength-high yield strength is unique to the materials of the present invention.

ice

In general, this invention comprises alloy steel compositions containing about (in weight percent) Percent Carbon 0.480.52 Manganese 0.75-0.95 Silicon 1.90-2.20 Tungsten 0.90-1.10 Vanadium 0.10-0.15 not more than 0.04 percent combined Phosphorus and Sulfur, balance Iron. A preferred composition may be defined as follows:

The lower combined phosphorus and sulfur content provides for weldability and improved toughness.

The alloys of this invention are preferably heat treated by austenitizing at 1600 F., oil quenching, refrigerating to F., double tempering for 1+1 hours at 775 to 800 F. In this heat treated condition the alloys of this invention are characterized by the following properties:

Ultimate tensile strength276,000 to 290,000 p.s.i.

0.2 percent offset yield strength2'60,000 to 274,000 p.s.i.

Elongation5 .5 to 6 percent Net fracture strength250,000 to 260,000 p.s.i. (0.050-

inch-long center crack).

The alloys of this invention meet other requirements of rocket motor casing material in addition to those set out above. For example, they can be rolled to 0.050- inch sheet and they are fabn'cable by existing forming and welding techniques. Since it is impossible, by standard fabrication methods for manufacturing rocket motor cases, to avoid sma'll flaws and defects that are not detectable by existing inspection techniques, a particularly outstanding characteristic of the present alloys is their ability to maintain their load-carrying ability despite these defects and flaws. This ability is reflected in their high fracture strength. 7

The following examples will illustrate alloy compositions of the present invention:

Example 1 An alloy having the following composition was pre- Specimens prepared from this :alloy exhibited ultimate tensile strengths ranging from 255,000 to 329,000 p.s.i. (depending on tempering temperature) with yield strengths ranging from 240,000 to 310,000 p.s.i. and elongations ranging from 4.5 to 7 percent. Net fracture strengths ranged from 230,000 to 260,000 p.s.i. for tempering temperatures from 750 to 850 F. For an alloy of this composition, tempered at 775 F., the mechanical properties were as follows:

Ultimate tensile strength p.s.i 290,000

Yield strength p.s.i 274,000

Elongation "percent" 5.5

Net fracture strength p.s.i 260,000

Example 2 An alloy having the following composition was prepared:

Percent C 0.48 Mn 0.82 Si 2.10 P 0.004 S 0.008 W 1.08 V 0.16 Fe Balance Ultimate tensile strength p.s.i 290,000 Yield strength p.s.i 275,000 Elongation "percent" 5.5 Net fracture strength p.s.i 255,000

New and useful ultrahigh-strength steel compositions capable of use as fuel containers for solid-propellant rocket motors, or for tension members in aircraft, or for other high-strength applications have been described. These alloys possess an unusual combination of yield strength, reduction in area, elongation properties, and fracture strength wherefore many other advantages and applications for these alloys will be apparent to those skilled in the art. It is desired to define the invention in the following claims which are intended to be interpreted, as to scope, in view of the description in this specification.

What is claimed is:

1. An alloy steel composition consisting of 0.48 to 0.52 percent carbon, 0.75 to 0.95 percent manganese, 1.90 to 2.20 percent silicon, 0.90 to 1.10 percent tungsten, 0.10 to 0.15 percent vanadium, not more than about 0.04

percent combined phosphorus and sulfur, balance essentially iron.

2. An alloy steel composition according to claim 1 characterized by an ultimate tensile strength in the range of 276,000 to 290,000 p.s.i., a 0.2 percent offset yield strength i in the range of 260,000 to 274,000 p.s.i., elongation in the range of 5.5 to 6 percent, and a net fracture strength L in the range of 250,000 to 260,000 p.s.i.

3. An alloy steel composition consisting of 0.48 to 0.52 'percent carbon, 0.75 to 0.95 percent manganese, 1.90 to 2.20 percent silicon, 0.90 to 1.10 percent tungsten, 0.10 to 0.15 percent vanadium, not more than about 0.015 percent combined phosphorus and sulfur, balance essentially iron.

4. An alloy steel composition according to claim 3 characterized by an ultimate tensile strength in the range of 276,000 to 290,000 p.s.i., a 0.2 percent offset yield strength in the range of 260,000 to 274,000 p.s.i., elongation in the range of 5.5 to 6 percent, and a net fracture strength in the range of 250,000 to 260,000 p.s.i.

5. An alloy steel composition consisting of about 0.49 percent carbon, about 0.82 percent manganese, about 2.14 percent silicon, about 0.004 percent phosphorus, about 0.006 percent sulfur, about 1.08 percent tungsten, about 0.14 percent vanadium, and the balance essentially iron.

6. An alloy according to claim 5 characterized by a net fracture strength of about 260,000 p.s.i. at a yield strength of about 274,000 p.s.i.

7. An alloy steel composition consisting of about 0.48 percent carbon, about 0.82 percent manganese, about 2.10 percent silicon, about 0.004 percent phosphorus, about 0.008 percent sulfur, about 1.08 percent tungsten, about 0.16 percent vanadium, balance essentially iron.

8. Alloys according to claim 7 characterized by a net fracture strength of about 255,000 p.s.i. at a yield strength of 275,000 p.s.i.

References Cited by the Examiner UNITED STATES PATENTS 7/ 1942 Shartell et al. -123 OTHER REFERENCES Allegheny A1 609 Alloy Digest, Filing Code: TS- 46 Tool Steel, May 1956, Published by Engineering Al-.

loys Digest, Inc., Upper Montclair, New Jersey.

DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, Examiner.

R. O. DEAN, P. WEINSTEIN, Assistant Examiners.

Dedication 3,293,028.A1"thur R. Elseu, Frank J. Barons, and William R. Warke, Columbus, Ohio. HIGH STRENGTH STEELS. Patent dated Dec. 20, 1966. Dedication filed May 7, 1973, by the assignee, The Battelle Development Gwyn/ration. Hereby dedicates to the People of the United States the entire renmining term of said patent.

[Ofiicz'al Gazette October 30, 1.973.] 

1. AN ALLOY STEEL COMPOSITION CONSISTING OF 0.48 TO 0.52 PERCENT CARBON, 0.75 TO 0.95 PERCENT MANGANESE, 1.90 TO 2.20 PERCENT SILICON, 0.90 TO 1.10 PERCENT TUNGSTEN, 0.1 TO 0.15 PERCENT VANADIUM, NOT MORE THAN ABOUT 0.04 PERCENT COMBINED PHOSPHORUS AND SULFUR, BALANCE ESSENTIALLY IRON. 